Here's how to determine the number of atoms in iodine pentachloride (ICl5):

* Identify the elements: Iodine (I) and chlorine (Cl)

* Determine the number of each element:

* There's one iodine atom (I).

* There are five chlorine atoms (penta means five).

* Total atoms: 1 + 5 = 6 atoms

Therefore, there are 6 atoms in one molecule of iodine pentachloride (ICl5).